Electrodeposition of tin



March 19, 1963 I H. H. FRANCISCO ETAL 3,

ELECTRODEPOSITION OF TIN Filed June 23, 1958 2 Sheets-Sheet 1 600 STRIP SPEED F 2: /M//v.

OXIDATION RA TE V6 Spesa Herberf h. Francisco Homer 6. Ress/er- Car/fan E. Rober-fs ,9 BY Richard G. Snyder LL T iORNEY March 19, 1963 Filed June 23, 1958 OXIDATION R4 753 A7 800 FZ/M/M (won/m: spa-0) H. H. FRANCISCO ETAL ELECTRODEPOSITION OF TIN 2 Sheets-Sheet 2 031089)!99 Na DINA/V45 J0 $97 INVENTORS Her-her) H. Francisco Homer 6- Bess/er Car/fan E. Roberfs Richard G. Snyder 'SATTORNEY United States P t Patented Mar. 19, 1963 3,082,157 ELECTRODEPOSIHON F TIN Herbert H. Francisco, Homer G. Ressler, Carlton E. Roberts, and Richard G. Snyder, all of Bethlehem, Pa., asfignors to Bethlehem Steel Company, a corporation of Pennsylvania Filed June 23, 1958, Ser. No. 743,845 14 Claims. 9c]. 204-54) This invention relatesto the electrodeposition of tin 10 from acid tinplating baths, and more particularly to electrodeposition from baths of the stannous fluoborate type.

One of the principal objects of this invention is to control the rate of oxidation in an acid tinplating bath.

Another object is to provide a bath having a low rate 5 of oxidation.

A further object is to maintain stannic tin in the soluble form in an acid tinplating bath.

Another object is to provide a bath with a high anode limiting current density and high cathode efiiciency.

In the development of high speed acid tin electro plating lines for the plating of steel strip, one of the serious drawbacks has been excessive oxidation in the plating bath.

In a vertical high speed line for the plating of strip, it is desirable to use a deep plating tank, with the strip passing through the tank in vertical skeins. The strip passes over a contact roll at the top of, and outside, the tank, down into the tank and under an idler roll at the bottom of the tank, then upward and over the next succeeding contact roll, proceeding from there to the next idler roll, and so on, in a series of slteins, numbering anywhere from two to thirty two or more. Squeegee, or back-up rolls, are usually placed adjacent the contact rolls on the side on which the strip leaves the bath. The

strip passes between the squeegee and contact rolls and, when travelling rapidly, carries considerable solution with it. When the liquid strikes the juncture between squeegee and contact roll, spray is formed, and this spray entraps air, both at the point of spray formation and as the spray falls into the bath. Contact of the spray with oxygen of the air causes oxidation of the tin in the bathspray, resulting in a build-up in stannic tin in thebath, as the oxidized spray is returned to the bath.

In addition, the faces of the contact and back-up rolls are often shot-blasted, or otherwise roughened, to aid in the traction of the rolls on the strip. The resultant indentations, or cavities, in the roll surfaces carry pockets of air into the line of tangency of the two rolls. The pocketed air is compressed and then dispersed under prjessure in the solution, which is carried to the rolls by the moving strip. The dispersed air results in an increised rate of oxidation of stannous tin to stannic tin.

In an acid tin plating bath the tin should be in'" the stannous, or bivalent state. Regardless of the nature of the tin salt which supplies the. stannous tin ions go thebath, oxidation of the tin takes place to some extent, the tin being oxidized from the stannous to the stannic con dition. The stannic tin has no utility insofar as producing platable tin ions is concerned, and when present in sufiicient quantity will precipitate, causing sludgiri'g in the bath. The resulting precipitate must eventually be removed from the bath, and it represents a loss of tin unless reclaimed. Reclaiming the precipitated tin adds considerable cost to the plating operation. if t l 1 'e rate of formation of stannic tin is high, thereis a s'erious. de-' pletion of stannous tin in the bath, requiringreplenishing of the stannous tin in the form of expense/e stannous compoud.

The presence of precipitated matter, particularly some forms of stannic tin compounds, in the batlfhas a deleterious eifect nu'submerged roll bushings, or seals, du

to-the somevgit abrasive nature of theprccipitate.

Most acid .plating lines can be operated satisfactorily,

insofar as oxidation problems are concerned, if the speed of the line is held below 400 feet per minute. However,

on a vertical type line, at speeds above 500 ft./min., the amount of aerated spray carriedinto the bath. causes oxi dation of 't he' tin at such a rapid rate that the cost of operation'pf the plating line becomes almost prohibitive.

Another' factor, in the rate of oxidation-of the tin, is the amoutof iron contamination in solution in the bath. Iron is introduced into the bath from the steel strip in the fcl't'tfils condition, and it apparently has a positive catalyticgeffect on the oxidation reaction.

In accordance with this invention, we have found that by proper selection of bath components and proper bath maintenance, we are able to control the rate of oxida- 'tion iriZan acid tin bath so that the bath may be operated economically on a vertical line at speeds of 900 ft./min. or gr ter.

Wc-have developed a bath which is substantially non slud ng, and we define the means for controlling the anionic bath concentrations as related to the stannous tin, tannic tin and incidental iron concentrations. We hav' wide operating limits for a bath which will be low in oxidation rate, yet will have a high anode limiting curfrent density, will be practically non-sludging, and will yield good tin plate.

Our bath is operable at or more cathode electrw chemical efiiciency, provided the balance between the cathode current density, stannous tin concentration and ath temperature are maintained according to known lectroplating principles. The stannous tin concentration :can be varied to obtain a high cathode eliiciency over a wide range of current densities. The cathode current efficiency is affected by other variables, such as free acidity, and, as is well known in the plating art, the

' proper conditions can be selected to obtain high cathode current efiiciency.

In a starting acid tinplating bath, the tin should be almost entirely in the stannous or bivalent state, and the concentration of the stannic tin is quite low. Starting, or make-up, baths can be made according to the examples given below.

Example I This is an example of a preferred starting bath, useful in the plating of steel strip.

We prefer to add (a), (b) and (c) in the order shown.

The quantities for ,(a), (b) and (0) can be varied over quite wide limits: 5 to 65 g./l. for (a) and (b), and 2.5-32.5 g./l. for (c). The ingredients (a), (b) and (c) are believed to react in the bath in the manner represented by the combined equation.

and should be added to the bath in the substantial ratio of 2 parts (a):2 parts (b):l part (c). Sulfuric acid in excess of the stoichiometric amount required to combine with sodium bifluoride and boric acid, to form fluoboric acid, can be used up to a total sulfate (80,) concentration of 65 g./l., depending on the concentration of stannic tin in the bath, as will be explained. Benefits occurring from the use of sulfuric acid in our bath within the prescribed limits are, higher reflectivity of the prodnot, less anode sludging and less roll staining.

'-In Example I the fluoboric acid (HBF formed by the above reaction, and as determined by the Nitron" analytical method is equal to 18.7 g./l., or approximately 75% of the theoretical amount. Stannous tin will require 1.48=14.8 g./l. of (BF ion, therefore there is 4 g./l. excess fluoboric acid present in the bath for the given amount of tin.

In using the nitron" method, referred to herein, we analyze for (BF ion by precipitation with nitron acetate, using essentially the procedure described by C. A. Wamser in the Journal of the American Chemical Society, vol. 73, January 1951, p. 411.

EXAMPLE II This example is for another form of satisfactory starting bath.

(a) Sulfuric acid (H 80 added as a water solution 35 (b) Sodium bifluoride (NaHF 35 (c) Boric acid (H3BO3) 17.5 Stannous tin (Sn++) 10 Grain refining agent 0.8

the ratio of (a), (b) and (c) should be substantially constant to conform with 2(a) :2(b) :l(c)

Stannous chloride bath 4.8 Stannous sulfate b 5.6 Bath of Example II 0.2 Bath of Example I 0.1

The amounts are empirical and were attained from -an accelerated laboratory test designed especially for use in measuring oxidation resistance. In this test we pass 1 liter of oxygen (dispersed by a medium porosity fritted disc) through 200 ml. of electrolyte at a constant rate of 2.2 ml./sec. The amount of oxidation is expressed in terms of stannic tin formed, where zero indicates almost complete absence of oxidation, and 0.1-0.2 g./l. of stannic tin (Sn++++) represents a very low oxidation rate.

A starting bath suitable for use with high current densities, such as those used in wire plating, is shown in Example III.

To calculate the amount-of fluoboric acid to be added to this bath, the nitron analytical method indicates that reaction between (a), (b) and (c) will yield 18.7 g./l. HBF The stannous tin will require 80 1.48=ll8 g./l. of 100% strength HBF while the desired excess of this acid is 10 -g./l., or a total of 128 g./l. As the make-up bath reaction produces 18.7 g./l. HBF it is necessary to 4 tadilh 128.0-18.7 or 109.3 g./l. HBF, for this high tin Most commercial fluoboric acid (HBF contains free boric acid, and we prefer not to have any free boric acid in our bath. Therefore, any fluoboric acid containing free boric acid, which is used in our bath, is first treated in the following manner:

For each mol of free boric acid (H B0 present, four mols of hydrofluoric acid (HF) are added. In additron, we add up to 2% excess hydrofluoric acid.

To sample a drum of fluoboric acid for use in the plating system, we weigh 50 grams of the concentrated acid from the drum, dilute to 1000 ml. with water, and let stand 30 minutes prior to analysis. This sampling technique is applied to all strong fluoboric acids, in order to attain a knowledge of the degree of hydrolysis that pertains to the dilution of fluoboric acid, in a 10 g./l. stannous tin fluoboric acid plating bath. The technique is purely arbitrary.

Typical analyses of a drum of fluoboric acid (drum contains 520 lb. net weight), before and after addition of HF, are given below:

Before Addition of HF Ingredient Weight After Addition of 50 lbs. of HF Ingredient None 'Iotal boron Total fluorine The hydrofluoric acid required, for modification of any particular lot of fluoboric acid, depends on the amount of excess boric acid present. The acid prepared in the above treatment, we designate as modified fluoboric acid. This modified fluoboric acid is used for all additions of fluoboric acid to the bath.

After the above treatment, the fluoboric acid is analyzed, and the necessary additions to the bath are made on the basis of the HBF, found by analysis.

EXAMPLE IV This is an example of a starting bath wherein all of the fluoboric acid is added when the bath is made up, rather than formed in the bath.

G./l. Stannous tin (Sn++) 10 Fluoboric acid (HBF 20 Grain refining agent 0.8

' in the module.

In maintenance of the bath, the fiuoborate determined by analysis is calculated to fluoboric acid (HRH). The following stoichiometry is used in calculating the fluoboric acid required, based on 100% strength HBF For each gram per liter of stannous tin, 1.48 g./l. of

l IB'F, is required.

For each, gram per liter of stannic tin, 2.96 g./l. of

HBF; is required.

For each gram per liter of ferrous iron, 315 g./l. of HBF, is required.

In addition to the above requirements, we desire an excess of from 5 to 15 g'./l. of HBF The bath is maintained by adding the correct amount (stoichiometric equivalent) of modified fluoboric acid, represented by the difference between calculated requirement, and the actual amount of HBF, present in the bath, as determined by analysis.

Laboratory tests were made using much broader ranges than those shown for the production run of Example V. These broader ranges for the individual components are given in Example VI.

; EXAMPLEVI Stannous tin (Sn++) 6-80 g./l; Stannic tin (Sn++++).'. 0-50 or more g./l. Ferrous iron (Fe' 0-10 g./l. Sulfate (S0,)- 0-65 g./l. Total fluorine (F) 13-350 g./l. Total boron (B) 1.5-37 g./l.

Fluoborate (BFQ- 12-300 g./l. Grain refining agent 0.2- g./l. Organic antioxidant 0-20 g./l. Temperature 40-90 C. Cathode current density 25-1000 a.s.f. or above.

For efiicient operation of the bath there should be at least several grams per liter excess fiuoborate in the bath, over that required to combine with the tin and iron. Excess fluoboroate may range as high as 100 g./l. or more. The concentration of sulfate ion is determined analytically, and the H 80 calculated. The sulfate is preferably maintained at from 33-37 g./l., and is added in modules where the module contains H 80 NaHF, and

' H,BO, in the ratioflof 2:2:1 respectively. A module will yield 0.53 pound of (BF,)- ion for each pound of H 80 This yield of (BF,)" ion must'be taken into accountin calculating HBF, additions.

The amount of sulfate tolerated by our bath depends somewhat on the stannic tin content. Anode limiting current density tests, whichwere made at 400 a.s.f.' and 70 0., proved satisfactory with a range of 0 to 30 g./l. stannic tin and 35 g./l. sulfate (S04), 0-20 g./l. stannic tin and 50 g./l. sulfate, and 0-10 'g./ l. stannic tin and 65 g./l. sulfate. Current densities other than 400 a.s.f.

will tolerate more or less sulfate, e.g. increasing the current density above 400 a.s.f. will lessen the tolerance, while decreasing the current density below 400 a.s.f. will permit greater concentrations of sulfate.

Both stannous tin and stannic tin can vary within rather wide limits, provided there is present the proper amount of fluoboric acid, calculated as shown above.

In the examples which we have given for starting baths, itwill be observed that sulfuric acid, sodium bifluoride and boric acid, where, present, are always given in the ratio of 2:2: I respectively. This is the desired ratio, although a departure therefrom of from 5 to 10% is permissible. Starting baths which are not initially made up from sulfuric acid, sodium bifluoride and boric acid, for instance, the bath of Example IV, and operating baths which do not contain the sulfate radical, are quite effectivein minimizing the rate of oxidation in the bath, but are less efficient than those baths which contain sulfate, and which are made up using the foregoing ratio.

When sulfate is present in the bath, analytical control is facilitated, for a simple sulfate determination will permit the calculation of solution lost, and from this, in turn, calculation can be made of the other bath constituents which have been lost through dragout, etc. While we have selected sulfuric acid, sodium bifluoride and boric acid as the reactants to produce (BF,,)- ion in the bath, other fluorides may be substituted for the sodium bifluoride, for example, ammonium bifiuoride (NH )H-F and sodium fluoride-hydrofluoric acid (NaF-HF). -Magr1esium fluoride (MgF,) may be substituted for sodium fluoride.

Our method of controlling an acid tin fiuoborate bath, by maintaining fiuoborate radical in the bath in an amount at least equal to the stoichiometric equivalent of the stan- -nous tin, stannic tin and ferrous iron, is applicable whether the bath contains an excess of hydrofluoric acid, or fluorine ion, or a small excess of free boric acid, or borate ion.

In the foregoing examples of starting baths, the boric acid is added in a quantity in which it will be completely reacted in the formation of fluoboric acid. Optimum results will be obtained when there is no unreacted boric acid in the operating bath, and to assure this condition,

an excess of hydrofluoric acid should be maintained in' the bath at all times. However, a small amount of boric acid can be tolerated in our system, so long as there is not enough to cause precipitation of bath ingredients, at a rate which would result in objectionable sludging. In the appended claims, the expression substantially free of excess borate radical" means that there is insufficient excess borate radical in the bath to produce objectionable sludging.

The net resultof the lowered-oxidation rate, characteristic of our bath and of our method of control, is the formation-of much less stannic tin than would normally be encountered in high speed plating. As only relatively small amounts of stannic tin are formed in our bath, this form of the tin is held in solution, presumably as a soluble fiuoborate. The excellent results we have obtained in the reduction of the oxidation rate of the bath, can be further improved upon by the; addition of an organic material having antioxidant properties, such as hydroquinone.

In addition, a compatible grain refining agent should be supplied to the bath.

In selecting a compatible grain refining agent, a number of the well-known addition agents have been found suitable, such as polyethylene oxide 4000, phenol sulfone,

cresol sulfonic acid, gelatine, glue and B-naphthol. Soluble quaternary ammonium compounds have proved especially eificient in our bath, particularly those having the general formula R: where n is an integer of from 5 to 10, R R, and R are alkane radicals, each of which comprises a member of the group consisting of methyl and ethyl, and A is an anion such as toluene sulfonate, iodide, fiuoborate, sulfate or the hydroxy radical. The aforesaid group includes such compounds as decyl t'riethylainmonium toluene sulf onate,

octyl trimethyl ammonium toluene sulfonateand decyl methyl diethyl ammonium iodide;

While our method of forming the bath has proved quite satisfactory from an operational standpoint, it is obvious that fluoborate radical may be introduced into the bath in a form other than the one described. For example, the bath may be made up with fluoboric acid (HBF or-with hydrofluoric acid (HF) and boric acid.

While we prefer to'use hydroquinone as the material by which we obtainan additional lowered oxidation rate, other antioxidants, such as catechol, para aminophenol, 4-aminoantipyrine and para methyl aminophenolsulfate, may be substituted.

While our method of forming the bath decreases the inception of oxidation, some oxidation of the stannous tin to the stannic form is inevitable due to the continual aeration of the bath and the presence of iron, which is carried into the bath by the steel strip. As the stannic tin (Sn++++) forms in an operating bath, 2.96 grams of fiuoboric acid (HBF are added for each gram of stannic tin. For each gram of ferrous iron introduced into the bath, 3.15 grams of fluoboric acid are added. In addition, for each gram of stannous tin (SN++) in the bath in excess of the make-up concentration, 1.48 grams of fluoboric acid are added. When sulfate is used in the starting bath, introduced either as H 80 or sodium sulfate (Na SO the sulfate radical may be held at a nearly constant figure during plating operations for control purposes. To make up for drag out, tin, sulfate radical, sodium ion, hydroquinone and the grain refining agent should each be added in the proportion used for the starting bath, and in amounts necessary to keep these ingredients at a desired operating level.

For optimum operating conditions, stannous tin should be maintained at around grams per liter. However, somewhat less tin, and up to about 80 grams per liter can be used without appreciably affecting the control of the oxidation.

Generally, in the operation of our. bath for strip plating, we prefer to maintain the temperature around 70 C. The current density may range from 50-800 a.s.f. for strip plating. Any type of conventional tin anode may be used. When plating at our preferred operating conditions, we develop a cathode efiiciency of 95% or above.

In order to prove the effectiveness of our treatment, a production run was made under typical operating conditions, in which approximately 1500 tons of tin-plated steel strip were produced. One-quarter pound plate was made in this run. The width of strip plated averaged 28.5 inches, and the plating was performed in a vertical system having an electrolyte volume of 15,000 gallons. The starting electrolyte for this test had the composition shown in Example 1. Operating conditions were as follows:

While operating the bath, it is of course necessary to make periodic chemical control checks in order to main; tain a fairly uniform bath composition. After determining bath requirements through such periodic checks, nee essary additions of sulfuric acid, bifluoride, boric acid, grain refining agent and hydroquinone were added to compensate for drag-out. Fluoborie acid was added in amounts necessary to satisfy the stannous tin and any stannic tin or ferrous iron which developed in the bath. All of the tinplate produced in the test run was subsequently fused in a radiant tube fusing furnace, and the quality of the resultant fused plate was excellent.

An outstanding characteristic of our bath is the degree to which oxidation of the tin is controlled. This oxidation control factor is very sharply illustrated by the accompanying drawing.

In the drawing, FIG. 1 is a graph representing an Tin (Sn++) 5 l... 10 Sodium fluoride (NaF) g./l. 44 Sulfuric acid (H g./l. 52 Grain refining agent g./l.-- 1.0 Operating conditions:

Temp. C 70 Current density .a.s.f.

These test runs were made on the same plating unit, and in the same manner, as used for the production run described above. The upswing of the curve, in the area defined by speeds of 500 to 600 ft./min., illustrates quite clearly the need for oxidation control for speeds above 500 ft./min. The sharp break in the curve, in the area referred to, is explained by the fact that above speeds of 500 ft./rnin. the intensity of back splashing of electrolyte from the contact and back-up rolls increases considerably.

In FIG. 2, curve A represents the oxidation rate in a bath made up as described for FIG. 1. The bath was run at a normal strip speed of 800 ft./min., and pounds of stannic tin have been plotted against base boxes of tin produced. This latter value is comparable to linear feet of strip plated. Curve B represents the oxidation rate for the bath used in the production run, the starting bath analysis of which is given in Example I. Oxidation figures were obtained from samplings of the electrolyte during the actual test run. It should be explained that while the runs represented by curves A and B were made at a normal strip speed of 800 ft./min., the strip speed was reduced to 400 ft./rnin. during such time as it was necessary to join one coil of strip to the next succeeding coil by welding. Thus, the strip speed was set at 400 ft./min. just prior to, and during, welding. After welding there was a certain time lag during which the strip regained its normal operating speed of 800 ft./min. It had been estimated that for any given coil, the speed was maintained at 800 ft./min. for about 70% of the coil length, the remaining 30% of its length being plated at a speed ranging between 400 and 800 ft./min. In compiling the data for both curves A and B, a series of a large. number of coils were plated in each instance, so that the 70/30 length ratio for 800/400 ft./min'. strip speeds is a good average.

Curve A, representing a tin bath in which no provision has been made to control the oxidation, shows a rapid rate of formation of stannic tin. Significantly, curve B, representing a bath controlled by our method, shows only a moderate rate of formation of stannic tin.

In our method, the concentration of the stannic tin reaches a point where said tin is removed from the bath, through drag-out, at a rate which is substantially equal to the rate at which it is formed in the bath, from which point thenceforth the stannic tin concentration remains constant. By maintaining the bath at this constant level, the concentration is well within the limits of economic operation.

While we have described our bath in conjunction with a vertical, or skein type, plating line, the bath has similar utility in a line of the horizontal, or tier, type.

We claim:

1. The method of controlling an aqueous electrolytic fluoboric acid tinplating bath of stannous tin in an amount of from 6-80 g./l., a grain. refining agent, and containing stannic tin and ferrous iron,-the plating metal of said bath consisting essentially of tin, which comprises maintaining fluoborate radical in the bath in an amount at least stoichiometrically equivalent to the stannous tin,

stannic tin and ferrous iron said bath being substantially free of excess borate radical.

2. The method of controlling an aqueous electrolytic fluoboric acid tinplating bath for the plating of a metal consisting essentially of tin, which comprises in a bath of stannous tin in .an amount of from 6-80 g./l., a grain refining agent, sulfate radical in an amount not in excess of 65 g./l. and containing stannic tin and ferrous iron, maintaining fluoborate radical in the bath in an amount at least stoichiometrically equivalent to the stannous tin, stannic tin and ferrous iron and maintaining the bath substantially free of borate radical.

3. The method of controlling an aqueous electrolytic fluoboric acid tinplating bath of stannous tin in an amount of from 6-80 g./l., a grain refining agent, sulfate radical in an amount not in excess of 65 g./ 1., an organic antioxidant in an amount of from 0.5-8 g./l. and containing stannic tin and ferrous iron, in which bath the plating metal consists essentially of tin, which comprises maintaining fiuoborate radical in the bath in an amount at least stoichiometrically equivalent to the stannous tin, stannic tin and ferrous iron and maintaining the bath substantially free of borate radical.

4. The method of controlling an aqueous electrolytic fluoboric acid tinplating bath of stannous tin in an amount of from 8-30 g./l., a grain refining agent and containing stannic tin and ferrous iron, the plating metal in said bath consisting essentially of tin, which comprises maintaining fluoborate radical in the bath in an amount at least stoichiometrically equivalent to the stannous tin, stannic tin and ferrous iron and maintaining the bath substantially free of borate radical.

5. A fluoboric acid electrolyte for depositing a metal consisting essentially of tin which comprises an aqueous solution of stannous tin in an amount of from 6-80 g./l., sulfate ion in an amount not in excess of 65 g./l., a grain refining agent, stannic tin, ferrous iron and substantially free of excess borate radical, and fluoborate radical in an amount at least stoichiometrically equivalent to the stannous tin, stannic tin and ferrous iron.

6. A method of preparing an aqueous electrolytic fluoboric acid tinplating bath of stannous tin in an amount of from 6 to 80 g./l., a grain refining agent and stannic tin, the plating metal from said bath consisting essentially of tin, which comprises introducing at least part of the required fiuoboric acid into the bath by adding sulfuric acid up to 6'5 g./-l., sodium bifluoride and boric acid in stoichiometric quantities necessary to produce fluoboric acid, adjusting the total fluoborate radical in the bath to an amount stoichiometrically equivalent to the tin plus an excess of at least 2 g./l. of the fluoborate radical by the additionof fluoboric acid said bath being substantially free of excess borate radical.

7. A method according to claim 4 in which the grain refining agent is decyl triethyl ammonium para toluene sulfonate.

8. A method according to claim 3 in which the organic antioxidant is hydroquinone.

9. A fluoboric acid electrolyte for depositing a metal consisting essentially of tin made by forming an aqueous solution of the reaction products of 5 to 65 g./l. of sulfuric acid, 5 to 65 g./l. of sodium .bifluoride, and 2.5

to 32.5 g./l. of boric acid in the ratio of about 2:2:1 respectively with 8 to 25 g./l. of stannous oxide, in which the fluoborate radical in the bath is present in an amount stoichiometrically equivalent to the stannous tin plus an excess of at least 2 g./l. fluoborate, and containing 2 to 10 g./ l. of hydroquinone and 0.3 to 10 g./l. of a grain refining agent, said electroylte being substantially free of borate radical.

10. An aqueous fiuoboric acid electrolyte for depositing a metal consisting essentially of tin which contains from 10 to 20 g./ 1. stannous tin, 30 to 40 g./1. sulfate radical, 17 to 104 g./l. total fluorine, 2.5 to 15 g./l. total boron, 20 to 120 g./1. fluoborate radical, 0.5 to 25 g./ 1. stannic tin, 0.1 to 1.0 g./l. ferrous iron, 0.2 to 10 g./l. of an organic grain refining agent and 0.2 to 8 g./l. of an organic antioxidant, said electrolyte being substantially free of borate radical.

11. An aqueous fluoboric acid electrolyte for depositing a metal consisting essentially of tin which contains from 6 to 80 g./1. stannous tin, 1 to g./l. sulfate radical, 13 to 350 g./l. total fluorine, 1.5 to 3.7 g./l. total boron, .12 to 300 g./1. fluoborate radical, 0.3 to 50 g./ 1. stannic tin, 0.1 to 10 g./ 1. ferrous iron, 0.2 to 10 g./ 1. of an organic grain refining agent and 0.2 to 20 g./ 1. of an organic antioxidant, said electrolyte being substantially free of excess borate radical.

12. The method of controlling an aqueous electrolytic fluoboric acid tinplating bath of stannous tin in an amount of from 6-80 g./l., sulfate radical in an amount not in excess of 65 g./l., a grain refining agent, stannic tin and ferrous iron, and substantially free of excess borate radical, the plating metal of said bath consisting essentially of tin, which comprises maintaining fiuoborate radical in the bath in an amount at least stoichiometrically equiva lent to the stannous tin, stannic tin and ferrous iron.

13. An aqueous fluoboric acid electrolyte for depositing a metal consisting essentially of tin comprising 6-80 g./l. of stannous tin and a grain refining agent, and containing stannic tin and ferrous iron as impurities, said bath containing finoborate radical inan amount at least stoichiometrically equivalent to the stannic tin, stannous tin and ferrous iron and being substantially free of borate radical.

14. The method of controlling an aqueous electrolytic fiuoboric acid tinplating bath of stannous tin in an amount of from 6-80 g./l., a grain refining agent, and containing stannic tin and ferrous iron, the plating metal of said bath consisting essentially of tin, which comprises maintaining fluoborate radical in the bath in an amount at least stoichiometrically equivalent to the stannous tin, stannic tin and ferrous iron, said bath being substantially free of borate radical.

References Cited in the file of this patent UNITED STATES PATENTS 2,421,079 Narcus May 27, 1947 2,446,716 Nachtman Aug. 10, 1948 2,461,350 Schaefer et al Feb. 8, 1949 2,846,381 Frick et al. Aug. 5, 1958 0mm REFERENCES Plating, vol. 4-2. :leptember 1955, pp. 1149-1150.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,082,157 March 19, 1963 Herbert H. Francisco et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 33, strike out "two", second occurrence;

line 69, for "compoud" read compound column 3, lines 1 and 2, for "occurring" read accruing column 5, line 21, for "315 g./l." read 3.15 g,/l, column 10, line 21, for "3. T g../1. read 3 7 g./l.

Signed and sealed this 8th day of October 1963.

(SEAL) Attest:

EDWIN L. REYNOLDS ERNEST W. SWIDER Attesting Officer Acting Commissioner of Patents 

1. THE METHOD OF CONTROLLING AN AQUEOUS ELECTROLYTIC FLUOBORIC ACID TINPLATING BATH OF STANNOUS TIN IN AN AMOUNT OF FROM 6-80 G./L/. A GRAIN REFINING AGENT, AND CONTAINING STANNIC TIN AND FERROUS IRON, THE PLATING METAL OF SAID BATH CONSISTING ESSENTIALLY OF TIN, WHICH COMPRISES MAINTAINING FLUOBORATE RADICAL IN THE BATH IN AN AMOUNT AT LEAST STOICHIOMETRICALLY EQUIVALENT TO THE STANNOUS TIN, STANNIC TIN AND FERROUS IRON SAID BATH BEING SUBSTANTIALLY FREE OF EXCESS BORATE RADICAL. 